1. Field of the Invention
The present invention relates to planar light source devices and display devices using the same, and particularly to planar light source devices using light emitting diodes and display devices using the same.
2. Description of the Background Art
As a planar light source device having a light emitting diode (LED) as a light source, a backlight of a liquid crystal display device used in a personal computer and the like has been conventionally well known. This planar light source device includes a light guiding plate for confining light, and a light source section. The light guiding plate is made of resin that is transparent and has a high refractive index, such as polycarbonate resin or acrylic resin. A light emission section is formed by mounting a plurality of LEDs on a substrate, and is arranged on opposite side surfaces of the light guiding plate.
In order to obtain stabilized luminance with this device, it is further required that a sensor be used to measure the quantities of light from the LEDs, and the quantities of light emission of the LEDs be adjusted by utilizing the results of measurements. For this reason, a planar light source device disclosed in Japanese Patent Application Laid-Open No. 11-260572 (1999) has a particular optical sensor provided on the end face of a light guiding plate, to adjust the quantities of light emission of LEDs serving as a light source by utilizing the results detected by the sensor.
When using LEDs as a light source like the planar light source device disclosed in Japanese Patent Application Laid-Open No. 11-260572, the outputs of the LEDs have strong temperature dependence, causing chromaticity and luminance to vary with a change in ambient temperature. The temperature dependences of LED outputs vary depending on colors of red (hereafter also indicated as R), green (hereafter also indicated as G), and blue (hereafter also indicated as B). Accordingly, a problem is encountered in the emission of white light by using LEDs of the three colors R, G and B such that a white balance varies with a change in ambient temperature. The light emission characteristics of LEDs vary among LEDs of the same color as well. Further, the life characteristics of LEDs also vary depending on the colors, so the range of variation in light emission characteristics of LEDs grows wider among the colors due to age deterioration.
In order to solve the above-discussed problems, the particular optical sensor detects variations in chromaticity and luminance of the LEDs caused by temperature changes and age deterioration, and the quantities of light emission of the LEDs are adjusted based on the detected results in Japanese Patent Application Laid-Open No. 11-260572. It has thus been required of conventional planar light source devices to include an additional particular optical sensor.
However, the provision of an additional particular optical sensor for a planar light source device involves increases in component count and the number of steps of the device, increasing the manufacturing cost. Further, the addition of an optical sensor, which is expensive in itself, increases the cost for the planar light source device itself.